Key West , Florida -LRB- CNN -RRB- -- For more than 20 years , the bulletproof museum case housed a small piece of yesteryear : a gold bar recovered from a sunken Spanish galleon . Today , its case is broken , littered with black fingerprint dust . The treasure is gone . Stolen . Two thieves were caught in the act by the museum 's security cameras .

`` This is a special piece , '' said Melissa Kendrick , executive director of the Mel Fisher Maritime Museum in Key West , Florida .

`` All the pieces have an incredible historic value , but this is the piece that was shared with the public in a whole totally different way . ''

It was different because visitors could touch it . By reaching into the specially designed display case , more than 6 million people have touched the 74.85-ounce bar , valued at more than $ 550,000 .

`` They 're touching something that belonged to someone in 1622 , '' said Carol Shaughnessy , author of `` Diving Into Glory . ''

`` Ordinarily people do n't get to touch something like that . You ca n't touch an Egyptian mummy . This is a hands-on connection to history . ''

But now , what does a thief do with a priceless , high-profile artifact ? Is there an underground market that will pay $ 550,000 for this almost 400-year-old piece of solid gold ? One expert says no .

`` That 's why these crimes do n't make a whole lot of money for the criminals , '' said Robert Wittman , a former FBI agent who once headed the FBI 's Art Crime Team .

`` It does n't make sense to do it . ''

Wandering through the museum , the thieves can be seen in security video trying to open museum doors . The video is incredibly clear . First , they appeared to be targeting a display case of gold chains . Then , after a security guard left this part of the museum , a man can be seen reaching into the case housing the gold bar and placing the little piece of history into his pocket before exiting the museum .

`` We 're getting information and following leads , '' said Key West Police Chief Donie Lee .

`` Unfortunately we have n't got the best lead , which is , I know that person and we go out , and it 's a positive ID , and we 're able to go out and pick those guys up . ''

What makes the crime so shocking , police said , is that the thieves were able to snap the glass at its edges . It 's not just any glass , but three-eighths-inch thick bulletproof Lexan glass .

`` By designating this as a handling object , it brought certain risks to the bar , '' Kendrick said .

`` But after your first five , and your next 10 , and when you get to 25 years , you start to get to the point when you think that it 's never going to happen . ''

Treasure hunter and salvor Mel Fisher recovered the solid gold bar from the wreck of the Santa Margarita in 1980 . Fisher and his team had been searching for the Nuestra Senora de Atocha and instead found the sister ship , the Santa Margarita . Both ships had gone down in a hurricane off Key West shortly after leaving Havana , Cuba , in 1622 .

The ships were headed home to Spain with a cargo of gold , silver and coins from the new world .

The team found the Atocha in 1985 . The stolen bar is one of dozens of gold and silver bars retrieved from the bottom of the sea .

Experts say that about 90 percent of stolen art and artifacts is eventually recovered that but it often takes years to find . The FBI has recovered more than 2,600 items of cultural property valued at more than $ 142 million . The items range from Colombian artifacts to Rembrandt paintings .

Wittman , the former FBI agent and author of `` Priceless : How I Went Undercover to Rescue the World 's Stolen Treasures , '' said the market is incredibly small for these high-profile objects . He said thieves often steal the items and then try to figure how to sell them .

`` We recovered paintings and artifacts that were missing for many years . Ten , 15 , sometimes 20 years , because the thieves could n't get rid of them , '' he said .

`` They kept them in their closets . They were white elephants . They made no money out of the deals . They were stuck . ''

In 1990 , thieves entered the Isabella Stewart Gardner Museum in Boston , Massachusetts , and stole 13 works of art , including three Rembrandts from the 1600s . None has been recovered , and federal agents are using DNA to try to find the perpetrators .

Wittman said no legitimate collector would take the risks associated with buying stolen goods .

`` They do n't buy stolen property , because ... they ca n't show it , they ca n't enjoy it . ... It makes them into criminals , and the last thing they want to do is spend a lot of money for a painting or for an artifact , whether it 's gold or whatever , and have it seized by the police and go to jail , '' he said .

Key West authorities said they believe the thieves were not locals and that they are probably long gone . The museum 's insurance company is offering a $ 25,000 reward for information leading to the return of the bar .

Police said they remain hopeful they will solve the crime but just hope they can recover this golden piece of history .

`` This is going to end up in somebody 's house probably , used as a paperweight , '' said Lee , who is leading the investigation .

`` Other than melting it down , which is the worst-case scenario for everyone , we 're just hoping that they will come to their senses somehow and return this back to the museum . ''

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Gold bar stolen from Key West , Florida , museum is worth more than $ 550,000

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Video shows crime taking place , but leads are sparse

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Thieves often ca n't sell stolen treasures because legitimate collectors wo n't bite

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But stolen treasures are often hard to find